If I take a photograph with a smartphone camera, how much information is there that could be used to identify the owner? I know of Exif tags and that all cameras have slight manufacturing variations that are (citation needed) able to be disambiguated in a lab setting. Other than those, what other information/metadata may be present?
Typically only EXIF data will be present in a smartphone or indeed any digital camera.
Of course, from EXIF data, you can identify the camera make and model and possibly the location of the image capture.
You are right in that in theory the differences in digitisers and post processing would enable at least the camera or lens to be identified by specialist processing even without the metadata.
If a specialist got hold of a smartphone/camera and some images, it is also likely that they would be able to match one to the other as well due to differences in the manufacturing though I don't know how reliable that would be.
In addition to the EXIF data, there is a number of indirect approaches you can take to gain some information, especially if you have a number of pictures.
All smartphones I know sequentially number their images. The filename leaks this information, allowing someone who has several of your pictures to know how many pictures you took inbetween. If you provide a series of pictures for any purpose, it is visible if there are gaps, i.e. if you left out any pictures. In a setting of evidence, that might be important.
The number also gives me an estimate of how many pictures you take, in general, disclosing your habit of photography or lack thereof. This may or may not be useful depending on context.
There can also be a thumbnail embedded in the EXIF data, and not all image manipulation software updates the thumbnail, potentially leaking information if a picture was cropped, rotated or mirrored.
And here is a link with a few more ideas that I found while checking the above: